Multiple wireless tags can be interrogated through sending from an interrogating transmitter (e.g., a reader) a code and having information transmitted by the tag in response. The interrogation is commonly accomplished through arranging the tags to listen for an interrogation message and to respond with a unique serial number and/or other information.
The tags typically have limited power available for transmitting data wirelessly to the reader. For example, in a reader-powered backscatter mode, the tags operate using the power of the received signal from the reader to transmit. In general, the tags may or may not have their own power source. It is generally desirable to extend the range of wireless tags so that it is not necessary to bring each tag close to a reader for reading. A reader is typically designed to have a high transmission power and high sensitivity to backscattered signals from the tags.
Traditional radio frequency identification (RFID) systems are typically bistatic systems or monostatic systems.
A bistatic system uses different antennae for transmission and reception. The antennae in a bistatic system are sufficiently separated in space to have fewer isolation problems. As transmit and receive are separate in space, an amplifier can be used on the receive side of the reader to improve sensitivity.
However, since a bistatic system uses distinct and separate receive and transmit antennae, a typical bistatic system has a large size and a high price. A typical antenna of an RFID reader system costs about $150 to $250. Thus, using distinct and separate receive and transmit antennae can lead to a high cost system.
A monostatic system uses the same antenna, or collocated antennae, for transmission and reception. When the same antenna is used for both transmission and reception, a monostatic system may use only half of the number of antennae that is used in a bistatic system and cover the same area. However, a monostatic system typically requires lots of tuning to isolate the transmit power and the receiver. In a typical RFID system, the transmit power of a reader may be around a Watt or two, while the receiver may be expected to be sensitive to signals at microwatt levels. Thus, it is difficult to implement a low noise amplifier (LNA) or a low noise receiver in a monostatic system. Since receive and transmit are at the same location, the spill over and cross talk can present a big challenge in the design of a monostatic system.
Conventional RFID readers are typically designed to use one of three general approaches to transmit signals to and receive signals from one or more tags, including a single-channel homodyne technique, a two-antenna bistatic technique, and a technique to use a circulator device.
FIG. 1A shows one standard approach for an RFID reader to read a beam-powered tag in which a homodyne receiver is used. The term “homodyne receiver” refers to the fact that there is but a single channel for both the transmitted signal and the received signal and a direct down conversion of the data to baseband. With the advantage of simplicity, a homodyne receiver is quite common. However, a homodyne receiver has the disadvantage of creating noise and lower sensitivity if not perfectly tuned.
In the example of FIG. 1A, the reader module (101) has a single antenna 107 coupled to both a radio frequency (RF) source (103) and a receiver (105). Using the signal from the frequency source (103), the receiver (105) directly down converts the data received from the antenna (107).
FIG. 1B shows an approach of a bi-static design where separate antennae are used for transmit and receive. For example, a reader module (121) in FIG. 1B has a radio frequency source (123) coupled to its own antenna (127) to transmit signals and a receiver (125) coupled to its own antenna (129) to receive signals. One disadvantage of a bistatic design is the added cost of two antennae instead of one. Since microwave antennae of some gain are expensive, and since the coax cable used at these frequencies can be expensive, the added cost of an additional antenna can be a major problem.
FIG. 1C shows a conventional technique to have a circulator in the reader to separate the incoming signal (to receive) from the outgoing signal (to transmit). A circulator couples the powers in a preferred direction so that the receiver retains backscatter information and the transmitter powers the tag. For example, the reader module (111) in FIG. 1C has a circulator (119) which couples power in a preferred direction, forward for transmit and power, and to the receiver for the receive or reflected portion. Power to the tag passes through to the antenna (117) and received power from the tag is channeled toward the receiver block after being reflected by the tag. In FIG. 1C, the circulator (119) couples port 2 to port 1 to transmit signals and couples port 2 to port 3 to receive signals. The use of a circulator in a monostatic design is also quite common but has the disadvantage of requiring a circulator that is an expensive device.
In a multistatic radar system, the transmitter and receiver antennae are positioned in different locations. The system is not confined to one receiver. Several receiver systems may be operated with one common transmitter. Alternatively, several transmit antennas can be used for transmit position or polarization diversity.